1. Field of Invention
This invention relates to polybenzimidazole articles and the process for their production. More particularly, the invention relates to substituted polybenzimidazole membranes that can be utilized for separatory devices and the process for their production.
2. Prior Art
In recent years there has been increasing interest expressed in microporous membranes of a semipermeable nature which are useful in separation techniques. Separation techniques which employ such membranes include electrodialysis, ultrafiltration, reverse osmosis, etc.
Semipermeable membranes prepared in the past have been formed from a variety of materials, and are characterized by the ability to permit a component (e.g., ions or molecules) of a solution to pass through a membrane to the substantial exclusion of other components (e.g., other ions or molecules). Examples of substances heretofore recognized to possess this property include cellophane (i.e., regenerated cellulose), cellulose esters (e.g., cellulose acetate, cellulose, butyrate, etc.), animal or protein membranes, polyelectrolyte complexes, ethyl cellulose, cross-linked polyacrylates, etc.
The semipermeable membranes of the prior art are of limited applicability in many separatory processes because of inherent disadvantages relating to their chemical stability, strength, thermal stability, efficiency, length of life and cost. See for example U.S. Pat. Nos. 3,133,132 and 3,567,632. Generally, the prior art membranes exhibit low thermal stability and therefore cannot be used successfully under conditions wherein the liquid undergoing treatment is at an elevated temperature. This may be a decided disadvantage in situations where the components to be separated only exist in solution at higher temperatures, or when it is economically advantageous to separate components of a solution at elevated temperatures rather than going through the expense of cooling the solution. Furthermore, some membranes exhibit a decided decrease in efficiency upon an increase in temperature or pressure thereby limiting their range of applicability. Solvent susceptibility may be another factor affecting the applicability of a particular porous membrane to a separation process. Additionally, semipermeable membranes may be inappropriate for a particular application due to low solute rejection values or low flux.
The chemical resistance properties of the prior art separatory membranes have been their greatest shortcoming. Although the separation of solutions comprising only sodium chloride and water present few chemical resistance problems to the prior art membranes, such pure solutions are rarely found. Many naturally occurring saline solutions contain materials which degrade previously known membranes. For example, cellulose acetate and amide-linked polymers, such as those disclosed in U.S. Pat. No. 3,567,632, are subject to either base or acid catalyzed hydrolysis even in weakly basic or acidic solutions. Other compounds which may degrade the prior art membranes include formic acid, acetone and bisulfite ions.
An additional problem of prior art membranes and ultrafilters has been their inability to filter over a broad range of flux and g/f.sup.2 /d. Many of the prior art membranes or ultrafilters could filter only within a very limited flux and g/f.sup.2 /d range.
In an attempt to overcome certain of the above-noted disadvantages of prior art membranes, membranes comprised of polybenzimidazole polymers have been provided. See, for example, U.S. Pat. Nos. 3,669,038; 3,720,607; 3,737,042; 3,841,492; 3,851,025; 4,020,142 and 4,512,894, each of which is incorporated herein by reference.
Typical processes for preparing polybenzimidazoles are described in the Journal of Polymer Science, Vol. 50, p. 511 (1961), and in various United States patents. U.S. Pat. No. Re. 26,065 describes a method for preparing high molecular weight, aromatic, polybenzimidazole polymers by melt polymerizing an aromatic tetraamine and a diphenyl ester or an aromatic, dicarboxylic acid anhydride at an elevated temperature and thereafter further polymerizing the product of the melt polymerization in the solid state. According to this process, in order to produce polymers of sufficiently high molecular weight to be suitable for practical use, it is necessary to pulverize the product of the melt polymerization prior to polymerization in the solid state and to conduct the solid state polymerization at an elevated temperature under a reduced pressure of less than 0.5 mm Hg or at an elevated temperature and in an inert gas stream over a prolonged period of time.
U.S. Pat. No. 3,313,783 describes another process which involves the solution polymerization in polyphosphoric acid of an inorganic acid salt of an aromatic tetraamine and a dicarboxylic acid, or a derivative thereof.
Another process for producing polybenzimidazoles is described in U.S. Pat. No. 3,509,108. In the process the monomers are initially reacted in a melt phase polymerization at a temperature above 200.degree. C. and a pressure above 50 psi. The reaction product is then heated in a solid state polymerization at a temperature above 300.degree. C. to yield the final aromatic polybenzimidazole product. The process requires the initial reaction to be conducted at a pressure above 50 psi (preferably, between 300-600 psi) in order to control the foaming encountered during the polymerization.
U.S. Pat. No. 3,555,389 describes a two-stage process for the production of aromatic polybenzimidazoles. The monomers are heated at a temperature above 170.degree. C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated in the presence of phenol to yield a polybenzimidazole polymer product.
U.S. Pat. No. 3,433,772 describes a two stage polymerization process for the production of aromatic polybenzimidazoles which utilizes an organic additive, such as an alkane having 11-18 carbon atoms or a polycarboxcylic hydrocarbon, in order to control foaming during the first stage.
Other United States patents relating to one-stage and two-stage production of polybenzimidazoles include U.S. Pat. Nos. 3,408,336; 3,549,603; 3,708,439; 4,154,919 and 4,312,976. All patents enumerated herein are incorporated by reference.
Polybenzimidazole polymers are valuable for the production of high temperature, stable, molded and extruded products, such as films and fibers. While polybenzimidazole articles have been quite useful as membranes and ultrafilters, they are by nature hydrophobic and, therefore cannot be wet with hydrophilic solvents. The hydrophobic character of the polybenzimidazole polymers limits the ability of these polymers for some applications, such as wet films and the like. Further, their range as ultrafilters is quite limited.
While polybenzimidazole membranes are generally resistant to chemical attack, they are sometimes subject to chlorine attack at the imidazole nitrogen-hydrogen bond. Substituted polybenzimidazole polymers are useful in solving this particular problem. U.S. Pat. No. 3,578,644 describes the preparation of hydroxyl modified polybenzimidazole by reacting a polybenzimidazole polymer with an omega halo alkanol or a 1,2-alkylene oxide in the presence of a basic catalyst such as sodium hydride. The use of an omega halo alkanol reactant requires a basic catalyst, and an inorganic salt is formed as a by-product. In addition, the use of a gaseous alkylene oxide reactant requires a pressurized vessel for the reaction medium. Further, the halo alkanol reaction does not produce hydroxyl substitution on some types of polybenzimidazole, such as poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole and other similar polybenzimidazoles where the bridging group between the reactive imidazole rings creates steric hindrance to the reactivity of the imidazole nitrogens.
Accordingly, it is an object of this invention to prepare modified polybenzimidazole polymer membranes.
It is a further object of this invention to prepare novel hydroxyalkylated polybenzimidazole polymer membranes.
It is a still further object of the present invention to prepare a substituted polybenzimidazole membrane wherein the flux rate can be controlled.
It is another object of this invention to prepare substituted polybenzimidazole polymer membranes with increased hydrophilicity.
It is a still further object of the invention to provide an improved semipermeable membrane which can serve as an ultrafilter for a broad range of molecular weight molecules.
These and other objects, as well as the scope, nature, and utilization of this invention, will be apparent from the following detailed description.